Benefits to participants and/or society. There are no direct benefits to participants. However, if clonidine is effective in blocking stress-induced and/or cue-induced craving, then the results will be used as a basis for designing a treatment trial, and drug abusers and society may benefit from the eventual use of clonidine or other alpha agonists for prevention of relapse in cocaine and heroin users. In addition, this research will provide information on the clinical relevance of a preclinical model of relapse, possibly strengthening arguments for its use in medication development.